The intensity of use of motor vehicles and the range of the transport jobs performed change during the many-year operation of such vehicles. The changes are introduced in result of ongoing analyses of actual current vehicle operation costs, reliability, and performance characteristics. The changes in the vehicle operation process, taking place with vehicle’s age, were analysed on the grounds of the mileage of over 9 000 motor trucks. The analysis covered a 20-year period of vehicle operation. The analysis results were used to estimate the mathematical models that describe the basic characteristics of the mileage growth process, with changes in these characteristics as observed in the recent years along with an intensive development of the road transport in Poland. Models of vehicle mileage growth have been developed for seven categories of engine cubic capacity. The coefficients of model equations have been given and the accuracy of the mileage values calculated on these grounds has been comprehensively evaluated. The relative measures of the scatter in the mileage values obtained from these models do not exceed 12 % of the average values determined from experimental data. A procedure has been proposed that leads to evaluating the mileage growth process and is based on the experience having already been gained in this field. It has been shown that the mileage growth process is strongly related to the engine cubic capacity. The mileage growth process is an important source of information for the planning of vehicle operation, forecasting of costs, and estimating of exhaust emissions and energy consumption in the whole cycle of operation of motor vehicles by transport companies